1. Field of the Invention
The present invention relates to an analog to digital converting device which converts the level of an arbitrary analog signal into a digital value every short sampling period of time and corrects the digital values to corrected values indicating the analog levels with high precision.
2. Description of Related Art
As an analog to digital (A/D) converter for converting the level of an arbitrary analog signal into a digital value every predetermined sampling period of time, generally-used A/D converters such as a ΔΣ type A/D converter, a successive approximation type A/D converter, a cyclic A/D converter and the like are well known. Recently, time A/D (TAD) converters have come to public notice. The sampling period of time in the TAD converters can be set to be shorter than that in the generally-used A/D converters. This TAD converter is, for example, disclosed in Published Japanese Patent First Publication No. 2004-274157.
For example, an accumulator battery (especially, a lithium ion battery) is mounted on a vehicle as a driving power source, and an A/D converter is used to detect the voltage of the accumulator battery from an analog signal indicating the battery voltage every sampling period of time. In this voltage detection, it is required to control each of the battery voltage and the remaining electric power of the battery within a proper range for the purpose of suppressing the deterioration of the battery. Because the battery voltage is greatly changed in a short time (e.g., in the order of millisecond), it is required to detect the battery voltage every shortened sampling period of time. Therefore, the TAD converter is useful to detect the battery voltage every shortened sampling period of time.
The TAD converter comprises a pulse delay circuit having a plurality of inverters connected with one another in a ring shape. The inverters receive the same analog signal set at a changeable input level to use the signal as the driving power of the inverters. The delay time of the inverters depends on the level of the signal. When a start pulse is inputted to the TAD converter, the pulse cyclically runs through the inverters while being delayed by the delay time in each inverter. The number of inverters, through which the pulse runs in a predetermined period of time, is detected. This number depends on the level of the signal. Therefore, when the pulse is inputted to the TAD converter every sampling period of time, the level of the signal is converted into a digital value corresponding to the detected number in an analog to digital converting characteristic every sampling period.
Although the generally-used converters have a linear analog to digital converting characteristic so as to linearly convert signal levels into digital values, the TAD converter has a nonlinear analog to digital converting characteristic so as to nonlinearly convert signal levels into digital values. Further, the TAD converter has a high temperature dependency, so that the digital value outputted from the TAD converter is largely varied with the atmospheric temperature. Therefore, the precision of digital values obtained in the TAD converter is lower than the precision of digital values obtained in the generally-used converters. To compensate for this low precision, in the Publication No. 2004-274157, each of a plurality of master reference voltages set at known levels is applied to the TAD converter to obtain a reference digital value, a correction equation is determined from the reference digital values such that corrected digital values, obtained by correcting the reference digital values according to the correction equation, linearly relate to the master reference voltages, and each digital value outputted from the TAD converter in response to the input level is corrected according to the correction equation. Therefore, the corrected digital values linearly depend on the input levels.
However, to determine the correction equation such that the corrected digital values linearly depend on the input levels, a plurality of circuits are inevitably required to apply the plurality of master reference voltages to the TAD converter. Therefore, the cost in the manufacturing of the TAD converter is considerably increased.
Further, the correction equation determined from a plurality of reference digital values is inevitably expressed by a polynomial. Therefore, an arithmetic circuit is inevitably complicated to correct digital values of the TAD converter according to the correction equation. Therefore, the size of the arithmetic circuit becomes large, so that the manufacturing cost is further increased.
These problems have generally occurred in A/D converters having a nonlinear analog to digital converting characteristic and A/D converters having high temperature dependency. In other words, these problems have occurred in any A/D converter which requires a plurality of master reference voltages to improve the precision lower than the precision in the generally-used A/D converters.